1. Field of the Invention
The invention relates to a double feed detecting apparatus for detecting double feed which is a phenomenon of transporting two or more sheet materials as they are lying one upon another, particularly by the utilization of an ultrasonic sensor, in an apparatus carrying a plurality of sheet materials thereon and having the function of separating and transporting the sheet materials one by one.
2. Related Background Art
In a scanner, a printer, a copying machine, a printer, an automated teller machine (ATM) or the like, there is provided a mechanism for separating and transporting sheet materials one by one. However, there is conceived the possibility that when only one sheet material should be transported, double feed which is a phenomenon of two or more sheet materials being transported while they partly or entirely overlapping one another occurs. Therefore, in an apparatus for transporting sheet materials, the function of detecting the double feed of the sheet materials becomes necessary. As a mechanism for detecting the double feed of sheet materials, a double feed detecting apparatus utilizing an ultrasonic is popular in each field. With bank notes (paper money) as an example of the sheet materials, description will hereinafter be made of a double feed detecting apparatus for detecting the double feed of the bank notes.
FIG. 20 of the accompanying drawings shows the epitome of a conventional bank note double feed detecting apparatus. In FIG. 20, the reference numeral 101 designates sheet materials to be detected, which herein are shown as bank notes. The reference numeral 102 denotes an ultrasonic transmitter which transmits an ultrasonic for the bank notes 101. The reference numeral 103 designates an ultrasonic receiver which receives the ultrasonic transmitted by the ultrasonic transmitter 102. Also, as shown in FIG. 20, the ultrasonic receiver 103 is installed so as to be opposed to the ultrasonic transmitter 102 with the transport path of the bank notes 101 interposed therebetween so as to be capable of receiving the ultrasonic transmitted through the bank notes 101. The reference numeral 108 denotes a wave analyzer which analyzes the reception signal of the ultrasonic received by the ultrasonic receiver 103.
Description will now be made of the operation of the double feed detecting apparatus shown in FIG. 20.
First, the ultrasonic transmitted from the ultrasonic transmitter 102 impinges on the bank notes 101, and the transmitted wave thereof is received by the ultrasonic receiver 103. Thereby, the ultrasonic receiver 103 outputs an output voltage varied in accordance with the reception intensity of the received ultrasonic. The wave analyzer 108 analyzes a variation in the output voltage output by the ultrasonic receiver 103 as an ultrasonic reception signal.
The double feed detecting technique of FIG. 20 is what is called a level determining process of detecting double feed by the wave analyzer 108 analyzing any change in the amplitude of the received ultrasonic. A double feed detecting method using this level determining process is described, for example, in Japanese Patent Application Laid-Open No. H5-40030.
The level determining process will be further described hereinafter. First, a double feed determining threshold value is set in advance, whereafter the bank notes 101 are transported and the ultrasonic receiver 103 obtains the amplitude of the ultrasonic transmitted through the bank notes 101. As compared with the amplitude of the transmitted ultrasonic when the bank notes 101 are properly transported one by one, the amplitude of the transmitted ultrasonic when the bank notes 101 are double-fed assumes a small value because the amount of attenuation of the ultrasonic becomes great. Accordingly, the amplitude of the ultrasonic obtained by the ultrasonic receiver 103 is compared with the aforementioned double feed determining threshold value in the wave analyzer 108, whereby it is possible to detect the double feed of the bank notes 101 from the result of the comparison.
In the level determining process according to the prior art, however, when the sheet materials transported are very thin, the amount of attenuation of the ultrasonic is small even if the sheet materials are double-fed, and as compared with a case where the sheet materials are properly transported, a great difference does not appear in the reception intensity of the ultrasonic and therefore, the determination of double feed detection becomes difficult.
Also, as another double feed detecting technique, there is a technique called a phase determining process of detecting the double feed of sheet materials by a phase variation in the waveform of a received ultrasonic. A double feed detecting method using this phase determining process is described, for example, in Japanese Patent Application Laid-Open No. 2000-211769.
To effect the detection of the double feed of sheet materials by this phase determining process, it is necessary to construct a complicated analog signal comparison circuit, and sampling data corresponding to a wavelength of a received signal waveform is obtained and at the same time, it is held in a storage device and the received signal is analyzed and therefore, a great burden is applied to a control circuit, and this leads to the problem that an integrated circuit (IC) for exclusive use and a control circuit having a high function become necessary.
Further, the above-described phase determining process suffers from the problem that the received ultrasonic signal is affected by changes in external factors such as the distance between sensors, the thickness of the sheet material, ambient temperature and humidity and the atmospheric pressure, and the accuracy of double feed detection is reduced. The phase determining process also suffers from the problem that when the sheet materials of which the double feed is to be detected are thick, the amplitude of the ultrasonic signal is greatly attenuated in case of double feed and the ultrasonic signal is hardly distinguishable from the noise signal of the ultrasonic receiver itself or an external device and thus, a waveform sufficient to enable the phase thereof to be determined may not be output.
FIG. 21 of the accompanying drawings shows another example of the conventional double feed detecting apparatus. In FIG. 21, the reference numeral 101 designates sheet materials to be detected, which herein are shown as bank notes. The reference numeral 102 denotes an ultrasonic transmitter which transmits an ultrasonic to the bank notes 101. The reference numeral 103 designates an ultrasonic receiver which receives the ultrasonic transmitted by the ultrasonic transmitter 102. Also, as shown in FIG. 21, the ultrasonic receiver 103 is installed so as to be opposed to the ultrasonic transmitter 102 with the transport path of the bank notes 101 interposed therebetween so as to be capable of receiving the ultrasonic transmitted through the bank notes 101.
The reference numeral 104 denotes a control circuit which supplies a pulse signal of 200 kHz as a transmitted ultrasonic signal to a drive circuit 105. The drive circuit 105 amplifies the pulse signal supplied from the control circuit 104 and outputs an ultrasonic pulse signal. Thereby, the ultrasonic transmitter 102 transmits an ultrasonic of 200 kHz on the basis of the amplified ultrasonic pulse signal. The transmitted ultrasonic signal supplied by the control circuit 104 is, for example, a signal which transmits a pulse signal of 200 kHz over a constant time for several periods. This is what is generally called a burst-wave, and the burst-wave is periodically transmitted once in several milliseconds (ms).
The reference numeral 106 designates an amplifier circuit which amplifies the received ultrasonic signal output by the ultrasonic receiver 103. This is because when the bank notes 101 which are to be transported come into between the ultrasonic transmitter 102 and the ultrasonic receiver 103, the ultrasonic signal transmitted from the ultrasonic transmitter 102 is attenuated, and becomes a very feeble signal before it reaches the ultrasonic receiver 103 and therefore, the received ultrasonic signal output by the ultrasonic receiver 103 becomes feeble in amplitude, and this signal must be amplified by the amplifier circuit 106 and be raised to a signal amplitude capable of effecting double feed detection judgment. The reference numeral 110 denotes a signal gain adjusting circuit 110 which adjusts the signal gain of the amplifier circuit 106. Here, the signal gain adjusting circuit 110 is e.g., a volume. Also, the adjustment of this signal gain is effected for individual products, for example, before the shipment of the products, or is effected by the user of the products.
The reference numeral 107 designates an A-D converter which converts the received ultrasonic signal (analog signal) amplified by the amplifier circuit 106 into a digital signal and outputs it to a signal analyzing circuit 108. The signal analyzing circuit 108 analyzes the received ultrasonic signal digitized in the A-D converter 107, and outputs the result of the analysis to the control circuit 104. The reference numeral 109 denotes a storage device which holds therein each set value of the double feed detecting apparatus shown in FIG. 21. Thereby, the double feed detecting apparatus shown in FIG. 21 performs a double feed detecting operation by the use of the set values held in the storage device 109.
Description will now be made of the operation of the double feed detecting apparatus shown in FIG. 21.
First, the ultrasonic transmitted from the ultrasonic transmitter 102 impinges on the bank note 101, and the transmitted wave thereof is received by the ultrasonic receiver 103. Thereby, the ultrasonic receiver 103 outputs a received ultrasonic signal varied in accordance with the reception intensity of the received ultrasonic. Next, the amplifier circuit 106 amplifies the received ultrasonic signal output by the ultrasonic receiver 103 at an amplification factor conforming to the adjustment by the signal gain adjusting circuit 110. Next, the A-D converter 107 converts the received ultrasonic signal amplified by the amplifier circuit 106 into a digital signal, and outputs the digitized received ultrasonic signal to the signal analyzing circuit 108. Next, the signal analyzing circuit 108 analyzes the digitized received ultrasonic signal output by the A-D converter 107. Next, the control circuit 104, when it judges double feed on the basis of the result of the analysis by the signal analyzing circuit 108, performs the process of informing the apparatus or the utilizer of the apparatus to the effect that double feed has occurred.
The double feed detecting method of FIG. 21 is the aforedescribed level determining process of analyzing a variation in the amplitude of the received ultrasonic signal by the signal analyzing circuit 108 to thereby detect double feed. This level determining process will be further described below. First, a double feed determining threshold value is set in advance, whereafter, the bank note 101 is transported and the ultrasonic receiver 102 obtains the amplitude of an ultrasonic transmitted through the bank note 101. As compared with the amplitude of the transmitted ultrasonic when the bank notes 101 are properly transported one by one, the amplitude of the transmitted ultrasonic when the bank notes 101 are double-fed assumes a small value because the amount of attenuation of the ultrasonic becomes great. Accordingly, the amplitude of the received ultrasonic signal obtained by the ultrasonic receiver 102 and amplified by the amplifier circuit 106 is compared with the aforementioned double feed determining threshold value in the wave analyzing circuit 108, whereby it is possible to detect the double feed of the bank notes 101.
As a problem in the conventional double feed detecting apparatus, there is the problem that differences in signal amplification factor and resonance frequency occur to each product due to the unevenness of the characteristics of the ultrasonic transmitter 102 and the ultrasonic receiver 103 and the unevenness of the constituent parts of the amplifier circuit 106 and therefore, it is necessary to adjust the signal amplification factors of individual products before the shipment of the products, and this leads to an increase in cost.
Also, there is the problem that even if as described above, the signal amplification factors of the individual products are adjusted before the shipment of the products, the signal intensity of the received ultrasonic signal is varied by changes in external factors such as the distance between the ultrasonic transmitter 102 and the ultrasonic receiver 103, the ambient temperature and humidity and the atmospheric pressure, and the accuracy of double feed detection is reduced.
Further, there is the problem that the burst transmission interval of an ultrasonic generating signal is always effected at a constant period and therefore, when an interceptor is inserted between the ultrasonic transmitter 102 and the ultrasonic receiver 103, whereby the reverberation of the ultrasonic is quickly attenuated, the transmission of the next burst-wave is waited for for an excess time, and this leads to the bad efficiency of burst transmission.